The present invention relates generally to systems for purifying exhaust gases from internal combustion engines. More specifically, the invention relates to particulate filters and methods of making particulate filters.
Combustion of fuel in an internal combustion engine produces particulates, such as soot, and other fuel combustion emissions such as carbon monoxide, hydrocarbons, and nitrogen oxides. Wall-flow particulate filters are often used in engine exhaust systems to remove particulates from the exhaust gas. Wall-flow particulate filters are typically made of a honeycomb-like substrate with parallel flow channels (also referred to as cells) separated by internal porous walls. Inlet and outlet ends of the flow channels are selectively plugged, such as in a checkerboard pattern, so that exhaust gas, once inside the substrate, is forced to pass through the internal porous walls, whereby the porous walls retain a portion of the particulates in the exhaust gas. In this manner, wall-flow particulate filters have been found to be effective in removing particulates from exhaust gas.
In addition to filtering particulates from exhaust gas, in some embodiments an oxidation catalyst may be applied to a filter to facilitate the oxidation of soot to CO2. In some applications, use of an oxidation catalyst to oxidize hydrocarbons and carbon monoxide, plus treatment of NOx, would also be desirable. However, in the latter case, incorporation of the oxidation catalyst function into the filter is not common practice because the filter substrate is generally of high thermal mass. The high thermal mass of the filter increases the time required for the filter to reach a temperature sufficient to “light-off” of the oxidation catalyst(s) and commence the oxidation of hydrocarbons and carbon monoxide, plus treatment of NOx, and therefore the efficiency of the catalyst function is decreased (i.e., insufficient quantities of hydrocarbons, NOx and carbon monoxide are catalyzed). To overcome this thermal lag time and increase oxidation of hydrocarbons and carbon monoxide, plus treatment of NOx, a higher concentration of precious metals (e.g. platinum group metals (PGM) such as platinum, palladium, or rhodium) is typically required to meet emission requirements. Therefore, rather than incorporating the hydrocarbon, NOx, and carbon monoxide oxidation catalyst functions into the filter, it is common practice to have a separate oxidation catalyst located upstream of the filter, where the hydrocarbon, NOx, and carbon monoxide oxidation catalyst is applied to a low thermal mass flow-through substrate. NOx catalysts are often placed downstream of the filter as well.
There is interest in incorporating additional catalytic functionality on a particulate filter. In particular, combining filtering and catalyzing functions may provide both weight and space savings, which is important for both passenger cars and commercial vehicles. Therefore the ability to tailor properties and microstructure of the filter substrate so as to best utilize and protect the catalysts, while maintaining the proper filtration efficiency and pressure drop is also of interest.